in the source for 0.90 alpha 1:

org.apache.fop.layoutmgr.inline.LineLayoutManager line 1481 reads

 currLM.getWordChars(sbChars, firstElement.getPosition());

This causes a NullPointer exception because currLM never gets a chance to be 
initialised. 

line 1466 reads 

if (firstElement.getLayoutManager() != currLM)

If currLM is still null, and firstElement.getLayoutManager() is also null, then 
currLM will not be initialised by the content of the if statement.

A quick fix seems to be replacing

- if (firstElement.getLayoutManager() != currLM)

with 

+ if (currLM == null || firstElement.getLayoutManager() != currLM)

This has cured the problem with my test.

I attach the FO file that reproduce the problem.
It could be that the syntax I used is not correct, in which case I would 
appreciate comments on how to do it bettwe.